The increasing demand for digital video/audio information presents an ever increasing problem of transmitting and/or storing an enormous amount of information. For example, the growth of the Internet and the proliferation of Internet browsers in recent years has enabled millions of users to access various information in a matter of seconds. Most of the information is currently in the form of text and/or still images and graphics. Generally, this type of information can be quickly downloaded and displayed by a user. More recently, audio and video clips or sequences have been added to numerous webs sites. Without an increase in bandwidth, accessing these clips requires more computational cycles and access time due to the sheer size of the image sequences.
Two methods of accessing an image sequence across a network are available. First, the user can download the compressed bitstream in its entirety, and then decode and display the image sequence locally using the user's computer. One drawback of this method is the length of time that a user must wait while the image sequence is being downloaded. Another drawback is that the user may need to reserve a large storage space to accommodate a large image sequence.
A second, and more practical, method is referred to as “video streaming” where the processes of downloading, decoding and displaying the image sequence can be performed simultaneously. Namely, each frame of a sequence is immediately decoded and displayed upon receipt by the user.
However, video streaming poses a new set of challenges. Generally, the pre-recorded original video sequence is compressed at a certain bit-rate. This bit rate may not coincide with the channel bit rate used by the user. Namely, users may download the image sequence at a different bit rate, depending on the available bandwidth, e.g., high user access time versus low user access time. Although the server can reduce the delivery rate of the image sequence to match the channel bit rate, such reduction will result in a “slow motion” display at the receiver.
Alternatively, one solution to address varying bit rate is to partially decode the image sequence at the server, and then re-encode the image sequence using different coding parameters, e.g., different quantizer scales, thereby resulting in a different bit-rate that coincides with the channel bit rate used by the user. This method has proven to be quite powerful, and accurate, for a reasonable range of bit-rates. Namely, this method effectively provides a “knob” with which the user can vary the bit-rate of the image sequence. Unfortunately, as the bit rate decreases significantly, the effect of using this “knob” is a significant degradation in the spatial quality of the image sequence. Furthermore, this “knob” would not be practical if the user wishes to maintain the spatial resolution at the expense of sacrificing the temporal resolution, e.g., reducing the frame rate from 30 frames/second to 15 frames/second.
Therefore, a need exists in the art for an apparatus and method for effecting dynamic frame rate control over a network.